The present invention relates to optical coherent communications, and in particular, feedforward data recovery for optical coherent communications.
Recent years have witnessed the emergence of commercial quantum communication systems and field test beds. Despite tremendous potential, however, the high costs associated with quantum communications have limited its application. Mainstream protocols, such as the BB84 protocol, are implemented with expensive, inefficient single photon detectors. These protocols require dedicated dark fibers for transmitting quantum signals because they cannot tolerate crosstalk from classical communication channels.
One proposed solution is the continuous-variable (CV) quantum encryption protocol, which is based on coherent detection. In coherent detection, a local oscillator (LO) is used to beat with the signal to be measured. The detection sensitivity can be greatly enhanced by increasing the power of the LO. In the meantime, only photons in the same spatiotemporal and polarization mode as the LO can be detected while noise photons in different modes will be suppressed effectively. Thus, the LO acts as a “mode selector” and can filter out most of the background noise. The “mode selection” function is important in protocols where the extremely weak quantum signal can be overwhelmed by background noise.
Coherent detection also relies on the fixed phase relation between the transmitter laser and the local oscillator laser. However, due to the lack of an effective way to precisely synchronize the frequency and phase of the transmitter laser with the local oscillator laser, phase noise is a major impairment in coherent optical communication. While various techniques, such as feedforward carrier recovery, optical phase-locked loops, and optical injection phase-locked loops, have been developed in classical coherent communications, these techniques are not applicable in quantum coherent communication protocols where the quantum signal is extremely weak and the tolerable noise (including phase noise) must be significantly below the shot noise limit. The current solution in quantum coherent communications is to generate both the quantum signal and the LO from the same laser and transmit both of them through the same communications channel. However, this arrangement can create serious security problems. In addition, this arrangement can reduce communication efficiency and multiplexing capacity since the strong LO (typically 7 or 8 orders brighter than the quantum signal) can generate both in-band and out-of-band noise photons when it propagates through the communications channel.
Accordingly, there remains a continued need for an improved phase recovery scheme for coherent optical communications. In particular, there remains a continued need for an improved phase recovery scheme which provides shot noise limited coherent detection without locking the phase of the transmitter laser to that of the LO laser, while also reducing infrastructure costs for quantum communications and potentially providing a vehicle for classical communications.